Improved viral screening assays and more intensive questioning of donors for high-risk behaviors have resulted in dramatic declines in the rates of transfusion-transmitted hepatitis and AIDS. Nonetheless, there is need for continued vigilance of the safety of blood supply. This study will enroll blood donors and prospectively followed blood recipients in order to: 1) establish ongoing surveillance of the incidence of breakthrough infections from transfusion-transmitted agents for which there are existing donor-screening assays (e.g. HBV, HCV, HIV, human T cell lymphotropic virus [HTLV]); 2) monitor the transfusion risk of established infectious agents that are not routinely screened in blood donors including CMV, EBV, parvovirus B-19, HHV-8 [Kaposi's sarcoma virus], and the recently described SEN and TT viruses (possible hepatitis agents); 3) establish a repository of linked donor and recipient samples so that any newly emerging infectious agent can be rapidly evaluated for its threat to the blood supply. The risk of these blood transmitted infectious will be assessed by molecular and serologic assays in adult patients at NIH and in children at Children's National Medical Center. Blood samples from recipients transfused on one occasion will be obtained pre and 4, 8, 12, and 24 weeks post-transfusion. Recurrently transfused patients will have additional samples at 16 and 20 weeks after the index transfusion and 24 weeks after the last eligible transfusion. After initial infectious disease testing, recipient samples and linked donor samples will be stored in a repository maintained by the National Heart Lung and Blood Institute. The availability of the repository will allow for the assessment of transfusion risk for newly emerging pathogens and also for known agents for which there is no practical assay currently available. For example, this would allow future testing for prions in new variant Creutzfeld-Jacob disease (human variant of mad cow disease) or testing for the trypanosome that causes Chagas disease. Informed consent will be obtained to store and later test samples in the repository. Testing will be limited to infectious agents that potentially threaten the blood supply. No genetic testing will be performed.